In an injection system of this kind, the pump unit and the injection nozzle form a unit. One unit fuel injector (UFI) per engine cylinder is incorporated into the cylinder head and driven by an engine cam shaft either directly via a tappet or indirectly via tilt levers.
In the unit fuel injectors known from the prior art, the control units are as a rule embodied as magnet valves. The valve actuation unit is embodied as an electromagnet that actuates the control valve. The magnet valve is open in the unexcited state. This provides a free flow from the pump unit to the low-pressure region of the system and thus enables filling of the pump chamber during the intake stroke of the pump piston as well as a return flow of fuel during the pumping stroke. Triggering the magnet valve during the pumping stroke of the pump piston closes this bypass. This leads to a pressure buildup in the high-pressure region and, after the opening pressure of the injection nozzle is exceeded, to the injection of fuel to the combustion chamber of the engine. The closing time of the magnet valve thus determines the onset of injection, and the closing duration of the magnet valve determines the injection quantity.
The UFI is a time-controlled injection system; that is, a mechanical connection between the onset of injection and the cam shaft position is lacking. The injection onset must therefore be associated as precisely as possible with a certain engine piston position or crank shaft position. To that end, an engine control unit is supplied with information on the engine piston position or crank shaft position. The electromagnet of the magnet valve is triggered for controlling the injection events in accordance with the chronological order stored in memory in the engine control unit and in accordance with the information obtained.
The known UFIs with control units embodied as magnet valves have the disadvantage, however, that typically magnet valves have a very long response time. The reason is that the magnet armature of a magnet valve, because of its mass, cannot be accelerated arbitrarily fast, since mass inertia forces are acting on it. In addition, the magnetic field must first be built up to generate the attraction force. Magnet valves are moreover relatively large in size and have a relatively large number of individual parts which must be assembled into the magnet valves in production. This is time-consuming and labor-intensive and makes the magnet valves quite expensive.